WO2000050769A1

WO2000050769A1 - Diffuser for a wind turbine

Info

Publication number: WO2000050769A1
Application number: PCT/NZ2000/000020
Authority: WO
Inventors: Miles Peter Fox; Trevor Andrew Nash; Derek Grant Phillips; Richard George James Flay; Gordon Draisey Mallinson; Peter John Richards
Original assignee: Vortec Energy Limited
Priority date: 1999-02-26
Filing date: 2000-02-25
Publication date: 2000-08-31
Also published as: NZ334382A; AU2702600A

Abstract

A Diffuser Augmented Wind Turbine has a nacelle (15) centrally supporting two or more rotor blades (not shown), the rotor blades being surrounded by a primary diffuser section (10) of substantially aerofoil shape when viewed in section and a much thinner aerofoil-like secondary diffuser (11) extending rearwardly from the trailing edge of the primary diffuser, wherein one or more slots (13) are arranged circumferentially around the diffuser assembly (10, 11) allowing the injection of air from the exterior surface into interior surface of the diffuser assembly (10, 11), the slots (13) further differentiating the primary and secondary diffuser sections (10, 11).

Description

DIFFUSER FOR A WIND TURBINE

FIELD OF THE INVENTION

This invention relates to a diffuser for a wind turbine, and has particular application to the design of a Diffuser Augmented Wind Turbine (abbreviated to DAWT").

BACKGROUND

A DAWT has a duct which surrounds the wind rotor blades and increases in cross-sectional area downstream of the blade plane. The increasing duct area downstream, results in a reduction in the mean velocity of the flow through the diffuser.

PRIOR ART

Examples of prior art DAWTs include:

GB 1.508 52A (BEN GURION UNIVERSITY OF THE NEGEV), 26 April 1978

US 4.166.596A (MOUTON. Jr. et al.) 04 September 1979

US 5,464,320A (FINNEY) 07 November 1995

Derwent Abstract Accession No. 87-035009/05, Class Q55, NL 8501618 (HAGA) 02 January 1987

OBJECT

It is an object of this invention to provide an improved diffuser for a wind turbine, or an improved Diffuser Augmented Wind Turbine, or one which will provide the public with a useful choice. STATEMENT OF INVENTION

In one aspect the invention provides a diffuser adapted to surround the rotor of a wind turbine, the diffuser having an interior surface and an exterior surface, a primary diffuser section and a secondary diffuser section, and a circumferential slot or slots in the diffuser allowing the injection of air from the exterior surface to the interior surface, wherein the primary and secondary diffuser sections together form a generally venturi-like shape at least along the interior surface of the diffuser.

Preferably the circumferential slot or slots separate the primary and secondary diffuser sections.

In use, wind enters the diffuser at the leading edge of the primary diffuser section, which is preferably of a diameter which is less than the diameter of the trailing edge of the secondary diffuser section.

In its most preferred form of the invention, the primary diffuser section is in the shape of an aerofoil (when viewed in cross-section). Though it will be appreciated that the complete primary diffuser section is in the shape of a duct or ring with the interior surface and exterior surface separated from one another because of the substantially aerofoil-like shape of this section.

Preferably the secondary diffuser section is also in the shape of an aerofoil (when viewed in cross-section). More preferably the secondary diffuser section is a thinner aerofoil than the aerofoil shape of the primary diffuser section. This will become apparent from the drawings illustrating various examples of this invention.

It is possible, though less preferable, to have a thin or substantially laminar secondary diffuser section shaped as a simple frustro-conical shape, i.e. as a straight, or curved, conical section, with its interior surface and exterior surface being substantially parallel to one another, i.e. formed from a single layer of material without any aerofoil shaping.

In its most preferred form of the invention, the primary diffuser section closely surrounds the turbine blades (ie. a slightly greater radius than the rotor blades), although it is possible to modify the diffuser to include an inner ring between the interior surface of the primary diffuser section and the tips of the rotor blades. This however is a much less preferred arrangement, as will become apparent from the following drawings.

In another aspect, the invention provides a Diffuser Augmented Wind Turbine having a nacelle centrally supporting two or more rotor blades, the rotor blades being surrounded by a diffuser as described above.

Preferably the nacelle is streamlined to assist in creating a venturi-like shape between the exterior surface of the nacelle and the interior surface of the diffuser, when viewed in cross- section.

Any number of rotor blades can be used, although in the following examples, four such rotor blades are described.

The invention lends itself to both small and large diameter DAWT's, the smaller DAWT's typically ranging from 1 metre to 5 metres with a typical size being a 3 metre diameter DAWT for use in stand alone power generation, with larger sizes from 5 metres diameter upwards, typically about 100 metres diameter, being designed for use in providing power to the national grid. The larger sizes can provide significant quantities of power, for example, with this invention a 54 metre diameter DAWT should provide 3.5 MW of power, whilst a 66 metre diameter DAWT should provide approximately 5 MW of power. For environmental reasons, a DAWT of up to about 54 metres diameter could be land-based, with larger diameters being used on floating off-shore islands, or fixed on base foundations below sea level.

These and other aspects of this invention which shall be considered in all its novel aspects, will become apparent from the following description, which is given by way of example only, with reference to the accompanying drawings, in which:

DRAWINGS

Figure 1 is a first example of the geometry of a preferred DAWT showing the relationship of the primary and secondary diffuser sections and the nacelle.

Figure 2 is a second example of a preferred DAWT showing the relationship of the primary and secondary diffuser sections and the nacelle.

Figure 3 is a third example of a preferred DAWT showing the relationship of the primary and secondary diffuser sections and the nacelle.

Figure 4 is a fourth example of a preferred DAWT showing the relationship of the primary and secondary diffuser sections and the nacelle.

Figure 5 is a fifth example of a preferred DAWT showing the relationship of the primary and secondary diffuser sections and the nacelle.

Figure 6 is a sixth example of a preferred DAWT showing the relationship of the primary and secondary diffuser sections and the nacelle.

Figure 7 is a seventh example of a preferred DAWT showing the relationship of the primary and secondary diffuser sections and the nacelle.

Figure 8 is another example of a modified DAWT showing a high included angle.

Figure 9 is an example of a modified DAWT showing a slightly less steep included angle and a differently shaped nacelle.

Figure 10 is similar to Figure 9 but with a bull nose style of nacelle.

Figure 11 is also similar to Figure 9 but with a mainly parallel nacelle body.

Figure 12 shows a computational fluid dynamic model (abbreviated to "CFD") drawing of a preferred DAWT having a 7 metre diameter rotor.

Figure 13 illustrates a cut-away view of the preferred DAWT of Figure 8, showing the nacelle and the primary and secondary diffuser sections, but omitting the rotor blades and supporting structures.

Figure 14 shows a section through a preferred primary diffuser.

Figure 15 shows a section through a preferred secondary diffuser. Figure 16 shows an alternative approach to the shape of the primary and secondary diffuser sections.

Figure 17 shows a modified DAWT having an inner ring between the turbine blade and the primary diffuser section.

Figure 18 illustrates the most preferred diffuser shape, similar to that shown in Figures 8 and 9, and how this shape can be defined mathematically.

PREFERRED EMBODIMENTS

Turning to Figures 1 -7, it will be apparent that the combination of the primary diffuser section 10 and the secondary diffuser section 1 1 when combined have an interior surface 12 approximating to the shape of a venturi. The secondary diffuser section 1 1 is separated from the primary diffuser section 10 by a circumferential slot, or slots 13. In Example 1, the secondary diffuser section 1 1 is straight, (i.e. it is not of an aerodynamic shape) whereas the primary diffuser section 10 has an aerodynamic shape, and has a substantially elliptical nose superimposed on two arcs meeting at the trailing end of the primary diffuser section.

The nacelle 15 is shown in half section above the centre line 16, such that the entire diffuser forms a ring around the central rain-drop shaped nacelle having a blunt forward end and tapered trailing end.

In Figure 1 the nacelle is preferably proportional to V20, whereas the turbine radius and the radius to the primary diffuser section is proportional to V7. (The notation V20, V7, etc. refers to our design calculations such that V20 refers to a DAWT having a rotor blade diameter of 20 metres, and a V7 refers to the calculations for a rotor blade diameter of 7 metres, and so on).

It will also be noted that in Figure 1, and throughout Figures 1-7, this sectional view of the nacelle and the diffuser, shows that there is also a venturi-like shape between the exterior surface of the nacelle and the interior surface of the diffuser.

Figure 2 illustrates a geometry similar to that of Figure 1, except that the nacelle 15 is larger, being a V7 sized nacelle. Figure 3 illustrates the inlet geometry for a V7 with the turbine radius extended to 3.90 metres with a slot width to primary diffuser of 0.180 metres.

Figure 4 illustrates an inlet geometry turbine arrangement similar to that of Figure 3, except that the nacelle is scaled to V20.

Figure 5 is similar to Figures 3 and 4, with the inlet geometry/turbine being designed to V20, with the area ratio set to 5 based on a turbine radius of 3.65 metres. The area ratio of the diffuser is 4, i.e. areas based on primary diffuser radius at axial location of turbine to exit radius of secondary diffuser. The secondary diffuser length is the same as the V7.

Figure 6 is similar to that of Figure 5, except that we have moved the slot between the primary and secondary diffuser up wind, to stop separation of the primary diffuser. This is done by keeping the area ratio the same as the inlet geometry/area ratio and increasing the length of the secondary diffuser.

Figure 7 is similar to Figure 6 with the reference length changed to 20 metres. This scales all lengths proportionally for a V20.

Figures 8-11 show a different family of designs with a much higher included angle than those shown in Figures 1-7. In particular Figure 8 exhibits the highest included angle of this family. It has a high inlet to exit area ratio with the possibility of increased power output. Figures 9-1 1 show higher included angles than Figures 1 -7 but slightly less steep included angles than that of Figure 8. The greater the area ratio the higher is the available power, but it must be determined against the shape's ability to keep its boundary layer attached.

Figures 8-10 show a part sectional view of the central nacelle 16 each being aerodynamically shaped to assist flow through the diffuser. As shown in Figures 8-10, the central nacelle has a contour based on a NACA aerofoil shape. (NACA stands for the US "National Advisory Committee for Aeronautics" and details of these NACA shapes can be found by reference to: (a) the NASA web site: http://naca.larc.nasa.gov/ , (b) an aeronautical textbook, or (c) by reference to a program such as NACA Aerofoils advertised on www.ctaz.com/~kelcomp/airfoils.htm).

Figure 11 on the other hand shows a part sectional view of a nacelle 16 having a substantially cylindrical main body 15. This cylindrical body 15 is simpler to build and is only slightly less effective in assisting air flow through the diffuser. Hence a cost-benefit study may show its advantage.

Numeral 11 in each of the Figures 8-11 represents the presence of a secondary diffuser (which is likely to have a much thinner aerofoil shape than the primary diffuser). The secondary diffuser 1 1 assists in increasing the inlet to exit area ratio, but in large diffuser diameters its better to stretch the main body, i.e. the primary diffuser and eliminate the secondary diffuser 11. The deeper, or thicker primary diffuser bodies shown in Figures 8-11 are helpful in large diameter models for engineering benefit due to the increased stiffness of the diffuser body.

Figure 12 is a Computational Fluid Dynamic model (CFD) for a preferred diffuser having both the primary and secondary diffuser sections of aerofoil shape. This plot shows the diffuser and the nacelle but without the rotor blades. There is no inner ring (i.e. this construction is similar to that of Figures 1 -7) so that there is a slot between the blade tip and the interior surface of the primary diffuser sections.

Figure 13 shows the preferred diffuser, in cut-away perspective view from a computer generated drawing of this diffuser.

Figure 12 shows a controlled inlet contraction of the flow into the blade plane, with a slot 13 between the blade tip and the primary diffuser walls. This gap allows the accelerated flow adjacent to the primary diffuser section to continue downstream of the blade plane and keep the flow attached to the diffuser walls.

Slot 13 between the primary and secondary diffuser sections directs flow tangentially along the second diffuser section. It should be noted that the exit area, i.e. the radius at the tip of the trailing edge of the secondary diffuser section is substantially greater than the available radius of the rotor blades, resulting in a substantial improvement in available air power.

In Figure 13, the slot between the primary diffuser section 10 and the secondary diffuser section 1 1 is shown at the top of the drawing, but the bottom of the cut-away view shows the primary and secondary diffuser portions merging and the presence of a flange 55 downstream of the plane of the turbine blades. The primary and secondary diffuser sections can be connected together at points around the circumference of the diffuser, and flanges such as flange 55 can be provided to enable the DAWT to be attached to a supporting structure (not shown).

An aerodynamic profile for both the primary and secondary diffuser sections enables the controlled flow contraction and diffusion and provides structural benefits for the design of this DAWT. In addition by using a streamlined shape for the nacelle, separation of flow downstream is minimised or substantially prevented.

We have also carried out wind tunnel model testing on a scaled down model which supports the CFD analysis of this design.

The diffuser of Figures 12 and 13, can be defined mathematically with reference to Figure 18.

The primary and secondary diffusers are each described by two equations for the outer surface and two for the inner surface with respect to the centreline. The nose of each diffuser has an elliptical form that joins tangentially with a circular arc to both the outer and inner surfaces. These arcs meet at the trailing edge of each diffuser. The equations used to describe the ellipse and circular arc are:

Ellipse iMϊ - Arc {z - c) + (z - c) = r²

where a,b,c,d are geometric constants for each curve r is the arc radius Each diffuser is characterised by the following parameters:

Throat Radius TR

Diffuser Inlet Area Ratio IARp

Diffuser Inlet Angle IAp IAs Diffuser Exit Area Ratio EARp EAR_S Diffuser Exit Angle EA_P EA_S Ellipse Thickness ETO_P; ETIp ETO_s, ETI_S Ellipse Length ELOp_, ETI_P ELO_s, ELI_S Primary Slot Length PSS Secondary Slot Length SSS_y, SS8_Z Inner Surface Radius ISRp ISR_s Outer Surface Radius OSR_p OSR_s Figure 18 shows a schematic cross-section of the primary and secondary diffusers. This cross- section is rotated at full revolution about the centreline to form the complete diffuser.

Figure 15 shows a typical section through a large size primary diffuser section 110 designed for a 30 metre turbine blade.

Figure 16 shows a typical section through the secondary diffuser for such a 30 metre diameter turbine blade. In both cases, in Figures 14 and 15 the diffuser sections are substantially hollow, having some form of outer cladding 120 supported on a light weight but strong interior structure 121. Both sections are substantially aerofoil in shape having a blunt almost tadpole-like nose, and a tapered trailing edge.

Figure 17 shows an alternative design, in which the primary diffuser section 20 and secondary diffuser section 21 are separated by a slot 22. Both are substantially aerofoil in shape, however the difference between the diffuser of Figure 16 and the diffuser of Figures 1-7 is that the primary diffuser section 20 has an interior surface 24 which for the most part is substantially flat. i.e. at least along that portion of the interior surface adjacent the tip of the rotor blades, whereas the primary diffuser sections shown in Figures 1-7 are more curved. We consider the variation shown in Figure 16 also to be of a venturi-like shape, we have found that this shape also works satisfactorily without an inner ring between the rotor blade and the interior surface of the primary diffuser section 20.

Figure 14 shows a less preferred version of the invention, in which there is an inner ring between the tip of the turbine blade and the interior surface of the primary diffuser section, in order to create an additional slot and hence injection of air along the interior surface of the primary diffuser section. In this drawing a nacelle 30 supports a plurality of rotor blades 31 which are separated from the interior surface of the primary diffuser section 32 by an inner ring 33, which provides an inlet slot 34. The primary diffuser section 32 is separated from the secondary diffuser section 35 by a secondary slot 36.

ADVANTAGES

Advantages of the DAWT and diffuser design for this invention are: (a) Preventing or minimising separation on either or both the primary diffuser wall and the nacelle.

(b) An improved power to weight ratio over a conical diffuser.

(c) The ability to scale this diffuser design for both large and small diameter DAWT's.

VARIATIONS

The slot between the primary and secondary diffuser sections may be a single continuous slot occurring between two spaced apart diffuser sections, or the diffuser sections may be joined together by one or more structural supports such that a plurality of slots around the circumferential boundary between the primary and secondary diffuser sections. In some cases the primary and secondary diffuser sections could be formed integrally with one another so shaped that there are a plurality of slots around this circumferential transition zone between what would then be called the primary diffuser section and the secondary diffuser section. However in most cases where the DAWT is of large diameter, i.e. about 5 metres in diameter, the primary diffuser section and secondary diffuser section are preferably constructed separately from one another, and then assembled to form the complete diffuser. In this specification, description of a supporting structure has been omitted, in order to explain the shape and flow characteristics of the diffuser. Any suitable supporting structure can be used.

In its most preferred form both the primary and secondary diffuser sections are of aerofoil shape, however it would be appreciated that the secondary diffuser section could be flat as in Figures 1- 7, rather than of an aerofoil shape as in the remaining figures. In most cases, there is no need for an inner ring between the rotor blades and the primary diffuser section. However as shown in Figure 14 such an inner ring may be used.

DAWT's in accordance with this invention may be made in a variety of sizes or shapes. A particular advantage of this design of the diffuser being that the diffuser length is relatively short allowing for compact and therefore more economical and light weight design than would be the case with a long conical diffuser. The design of the diffuser of this invention lends itself to both small and large DAWT's in both land-based and water-based environments. Finally, various other alterations or modifications may be made to the foregoing without departing from the scope of this invention.

Claims

1. A diffuser adapted to surround the rotor of a wind turbine, the diffuser having an interior surface at least part of which faces towards the rotor, and an exterior surface, wherein the diffuser has a generally venturi-like shape at least along the interior surface of the diffuser, and one or more slots in the diffuser allowing the injection of air from the exterior surface to the interior surface.

2. A diffuser as claimed in claim 1, wherein the one or more slots are arranged circumferentially around the diffuser to differentiate it into a primary diffuser section and a secondary diffuser section.

3. A diffuser as claimed in claim 1, wherein the leading edge of the diffuser has a diameter which is less than the diameter of the trailing edge of the diffuser.

4. A diffuser as claimed in claim 1 , wherein the primary diffuser section is in the shape of an aerofoil (when viewed in cross-section).

5. A diffuser as claimed in claim 1 , wherein the secondary diffuser section is for the most part thinner than the primary diffuser section.

6. A diffuser as claimed in claim 1 , wherein the primary and secondary diffusers are each described by two equations from the outer surface and two for the inner surface with respect to the centreline.

7. A diffuser as claimed in claim 6, wherein the nose of each diffuser has an elliptical form

defined by the equation : I — ) + — j = 1

where: a and b are geometric constants for each curve.

8. A diffuser as claimed in claim 7, wherein the rear of each diffuser is defined by an inner surface arc and an outer surface arc of different radii and the rearmost portions of the elliptical nose join tangentially with each arc.

. A Diffuser Augmented Wind Turbine having a nacelle centrally supporting two or more rotor blades, the rotor blades being surrounded by a diffuser as claimed in claim 8.

10. A Diffuser Augmented Wind Turbine as claimed in claim 9, wherein the nacelle is streamlined to assist in creating a venturi-like shape between the exterior surface of the nacelle and the interior surface of the diffuser when viewed in cross-section.